Converting a diaphragm electrolytic cell to a membrane electrolytic cell

ABSTRACT

Disclosed is a method for converting a diaphragm electrolytic cell to a membrane electrolytic cell by using the standard diaphragm cell equipment and applying a membrane over top of a matting material upon the cathode surface. This method permits a manufacturer having diaphragm electrolytic cells to convert those cells to membrane electrolytic cells without significant capital expenditure to achieve the desirable characteristics of a membrane electrolytic cell.

CROSS REFERENCE TO RELATED APPLICATION

This is a divison of application Ser. No. 790,756, filed Apr. 25, 1977,now U.S. Pat. No. 4,112,149.

This application is a continuation-in-part of U.S. patent applicationSer. No. 688,842 filed May 21, 1976, now U.S. Pat. No. 4,036,728.

BACKGROUND OF THE INVENTION

The present invention relates generally to the conversion of a standarddiaphragm electrolytic cell which is being used for chlorine and caustic(sodium hydroxide) production, to an electrolytic cell having a membranefor the same type of chemical production with the inherent advantageouscharacteristic of a membrane electrolytic cell. More particularly thepresent invention relates to a method for forming a membrane over astandard diaphragm electrolytic cell cathode by vacuum forming a mattingmaterial onto the foraminous electrode and subsequently applying amembrane material over top of the matting material which is fused into athin and uniform substantially hydraulically impermeable film. Such amethod will allow manufacturers having standard diaphragm electrolyticcell equipment in current use to convert that equipment to membraneelectrolytic cells with a smaller capital expenditure to yield a savingsin the operational costs associated with the use of membraneelectrolytic cells.

Electrochemical methods of manufacture are becoming ever increasinglyimportant to the chemical industry due to their greater ecologicalacceptability, potential for energy conservation, and the resultant costreductions possible. Therefore a great deal of research and developmenteffort is being applied to the electrochemical processes and thehardware for these processes. From this effort has come technologicaladvances such as the dimensionally stable anode and various coatingcompositions therefor which permit ever narrowing gaps between theelectrodes, such that the electrolytic cell has become more efficientfor use in electrochemical processes. Also the hydraulically impermeablemembrane has added a great deal to the potential use of electrolyticcells in terms of the selective migration of various ions across themembrane surface so as to exclude contaminants from the resultantproduct thereby eliminating some costly purification and concentrationsteps of processing.

One significant commercial possibility for these advances inelectrolytic cells would be in chlorine and caustic production. Chlorineand caustic are essential and large volume commodities which are basicchemicals required by all industrial societies. They are produced almostentirely electrolytically from aqueous solutions of alkali metalchlorides, with a major proportion of such production coming fromdiaphragm-type electrolytic cells. In the diaphragm cell process, brine(sodium chloride solution) is fed continuously into the anodecompartment and flows through the diaphragm usually made of asbestos,backed by the cathode. To minimize back migration of the hydroxide ions,the flow is always maintained in excess of the conversion rate so thatthe resulting catholyte solution has unused alkali metal chloridepresent. The hydrogen ions are discharged from the solution at thecathode in the form of molecular hydrogen gas. The cathode solution,containing caustic, unreacted sodium chloride, and other impurities,generally has been concentrated and purified later to obtain amarketable sodium hydroxide commodity and a sodium chloride which can bereused in a chlorine and caustic electrolytic cell for furtherproduction of sodium hydroxide.

The dimensionally stable anode is today being used by a large number ofchlorine and caustic producers but the extensive commerical use of thehydraulically impermeable membrane has been at least in part militatedagainst by the substantial capital cost involved in converting fromdiaphragm electrolytic cells to membrane electrolytic cells. This iscaused by the difficulty in placing a more or less planar membrane ontothe cathode assembly which is generally a three dimensional assemblyonto which the asbestos diaphragm is placed by vacuum forming from aslurry. The diaphragm has been improved by adding to the slurry fromwhich the diaphragm is desposited onto the cathode assembly, a polymericmaterial to act as a binding substance so as to improve the chemicalresistivities of the diaphragm material. This dimensionally stablepolymer modified diaphragm though is not a hydraulically impermeablemembrane. Another approach has been to form a membrane on the electrodesurface itself. The problem with this approach is that most of thecathode assemblies in current use are foraminous in nature and suchporosity makes it very difficult to deposit a membrane material to forma film directly onto the cathode assembly.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a methodfor forming a membrane over a standard diaphragm cell electrode assemblyso as to eliminate the substantial capital costs involved currently forconverting a diaphragm electrolytic cell to a membrane electrolyticcell.

It is another object of the present invention to provide a method forforming a membrane over a standard diaphragm cell electrode assembly sothat the advantages of a membrane may be realized by using existingdiaphragm electrolytic cell equipment.

These and other objects of the present invention, together with theadvantages thereof over existing and prior art forms which will becomeapparent to those skilled in the art from the detailed disclosure of thepresent invention as set forth hereinbelow, are accomplished by theimprovements herein described and claimed.

It has been found that a method for forming a membrane over a standarddiaphragm cell foraminous electrode can comprise the steps of:suspending a matting material in a liquid medium; inserting a foraminouselectrode into the suspension; vacuum forming a layer of the suspendedmatting material over the surface of the foraminous electrode so as tosubstantially reduce the porosity of the foraminous electrode material;applying to the surface of the foraminous electrode with the mattingmaterial thereon a layer of thermoplastic material including NAFIONparticles in the sulfonyl fluoride form; baking the foraminous electrodewith the layers in place until the thermoplastic material is fused intoa thin and uniform film on the surface of the matting material which issubstantially impermeable to hydraulic flow; and hydrolyzing the NAFIONto change the sulfonyl fluoride form to the cation exchange sulfonicacid form.

It has also been found that a membrane separator for a standarddiaphragm electrolytic cell can comprise: a standard diaphragm cellforaminous electrode; on the surface of the standard diaphragm cellforaminous electrode, a layer of matting material of such thickness asto substantially reduce the porosity of the standard diaphragm cellforaminous electrode; and on the surface of the matting material a thinand uniform hydraulically impermeable cation exchange membraneconsisting essentially of a film of copolymers having the repeatingstructural units of the formula: ##STR1## wherein R represents the group##STR2## in which R¹ is fluorine or perfluoroalkyl of 1 to 10 carbonatoms; Y is fluorine or trifluoromethyl; m is 1, 2 or 3; n is O or 1; Xis fluorine, chlorine, or trifluoromethyl; X¹ is X or CF₃ --CF₂ --_(a)O--; a is O or integer from 1 to 5; and the units of the formula (1)being present in an amount to provide a copolymer having a --SO₃ Hequivalent weight in the range of 800 to 1600.

A method for forming a membrane over a standard diaphragm cellforaminous electrode may comprise the steps of: suspending a mattingmaterial in a liquid medium; inserting a foraminous electrode into thesuspension; vacuum forming a layer of suspended matting material overthe surface of the foraminous electrode so as to substantially reducethe porosity of the foraminous electrode material; applying to thesurface of the foraminous electrode with the matting material thereon alayer of thermoplastic material including material containing carboxylion exchange groups; and baking the foraminous electrode with the layersin place until the thermoplastic material is fused into a thin anduniform film on the surface of the matting material which issubstantially impermeable to hydraulic flow.

A membrane separator for a standard diaphragm electrolytic cell cancomprise: a standard diaphragm cell foraminous electrode; on the surfaceof the standard diaphragm cell foraminous electrode, a layer of mattingmaterial of such thickness as to substantially reduce the porosity ofthe standard diaphragm cell foraminous electrode; and on the surface ofthe matting material a thin and uniform hydraulically impermeable cationexchange membrane consisting essentially of a film of copolymers havingthe repeating structural units of the formula: ##STR3## wherein Rrepresents the group ##STR4## in which R¹ is fluorine or perfluoroalkylof 1 to 10 carbon atoms; Y is fluorine or trifluoromethyl; m is 1, 2 or3; n is O or 1; X is fluorine, chlorine, or trifluoromethyl; X¹ is X orCF₃ --CF₂ --_(a) O--; a is O or integer from 1 to 5; and wherein R² isan ion exchange group selected from the group of oxy acids, salts oresters of carbon, nitrogen, silicon, phosphorous, sulfur, chlorine,arsenic, selenium or tellurium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The membrane separator for a standard diaphragm electrolytic cellelectrode assembly and the method for forming such a membrane willovercome many of the disadvantages of the prior art forms listed aboveand yield the benefits of the use of a membrane in an electrolytic cellwithout the substantial capital cost associated heretofore with theconversion of a diaphragm electrolytic cell to a membrane electrolyticcell. Most of these diaphragm electrolytic cells in use today are of twogeneral types. Both consist of an outer steel shell either cylindericalor rectangular which supports a cathode of perforated iron plate orwoven iron screen inside of the shell, generally referred to as aforaminous electrode element. This constitutes the cathode assembly. Theactual cathode surfaces are generally lined with a layer of asbestoseither in the form of paper wrapped around it or vacuum depositedfibers. The type of cathode assembly for which the present invention isespecially useful is that known as the Diamond Shamrock Cell wherein thecathode assembly consists of a rectangular steel shell housing with aninner assembly of lateral rows of vertically flattened wire-screentubes, upon which the diaphragm has been deposited by suction from acell liquor suspension of asbestos fibers.

Since these foraminous electrode assemblies generally have a highporosity it is necessary to reduce the porosity by vacuuming some typeof matting material onto the foraminous electrode surface beforeapplying a membrane material. The matting material may be an asbestossupport made from chrysotile asbestos fibers mixed with 5% (by weight)fluorinated ethylene propylene copolymer particles, or any othermaterial which will form a sufficient mat upon the foraminous electrode.Another example would be a cellulosic material. Alternatively, sheets ofmaterial such as filter paper could be wrapped around the electrodetube. It is believed that the exact nature of the matting material isnot of great significance since it is generally of a temporary naturefor the purpose of supporting the polymeric materials to form a filmupon the foraminous electrode. It is believed that any depositable fiberwill serve as an adequate support structure, inertness to chlorine cellenvironments not being necessary. Since the thickness of the supportstructure affects the cell potential it is desirable to obtain thethinnest matting structure consistent with the purpose of substantiallyreducing the porosity of the foraminous electrode material. One way ofbuilding a matting which is often used in industry is to suspend thematting material in a fluid medium and in the case of the asbestosfibers usually the cell liquor. The foraminous electrode material maythen be suspended into the slurry of matting material and a vacuumpulled to the inside of the foraminous electrode material such that thefibers of the matting material will be drawn onto the surface of theforaminous electrode. This support structure will then provide a uniformsurface on which the slurry of thermoplastic materials including NAFIONcan be applied. Once the thermoplastic material has been applied andfused, the support structure is no longer necessary and the filmperforms like a membrane. The matting structure itself must have a lowenough porosity as to retain particles in the range of less than onemicron on the surface without being pulled to the interior portions ofthe matting material.

The NAFION material being used in the present invention is a fluorinatedcopolymer having pendent sulfonic acid groups. The fluorinated copolymeris derived from monomers of the formula

    CF.sub.2 =CF--R--.sub.n SO.sub.2 F                         (1)

in which the pendent --SO₂ F groups are converted to --SO₃ H groups, andmonomers of the formula

    CR.sub.2 =CXX.sup.1                                        ( 2)

where R represents the group ##STR5## in which R¹ is fluorine or perfluoroalkyl of 1 thru 10 carbon atoms; Y is fluorine or trifluoromethyl;m is 1, 2 or 3; is O or 1; X is fluorine, chlorine or trifluoromethyl;and X¹ is X or CF₃ --CF₂ --_(a) O--, wherein a is O or an integer from 1to 5.

This results in copolymers having to repeating structural units ##STR6##

In the copolymer there should be sufficient repeating units according toformula (3) above, to provide an --SO₃ H equivalent weight of about 800to 1600. Materials having a water absorption rate of about 25 percent orgreater are preferred since higher cell potentials at any given currentdensity are required for materials having less water absorption.Similarly, materials having a film thickness (unlaminated) of about 8mils or more require higher cell potentials resulting in a lower powerefficiency.

Polymeric materials of this type are further described in the followingpatents which are hereby incorporated by reference: U.S. Pat. Nos.3,041,317; 3,282,875; 3,560,568; 3,624,053; 3,718,627; and BritishPatent No. 1,184,321. Polymeric materials as aforedescribed areavailable from E. I. duPont deNemours & Co. under the trademark NAFION.

Polymeric materials according to formulas 1 and 2 can also be madewherein the ion exchange group instead of being a sulfonic acid exchangegroup could be many other types of structures. One particular type ofstructure is a carboxyl group ending in either an acid, and ester or asalt to form an ion exchange group similar to that of the sulfonic acid.In such a group instead of having SO₂ F one would find COOR² in itsplace wherein R² may be selected from the group of hydrogen, an alkalimetal ion or an organic radical. These polymeric materials are availablepresently from E. I. duPont deNemours & Co. Furthermore it has beenfound that a substrate material such as NAFION having any ion exchangegroup or function group capable of being converted into a ion exchangegroup or a function group in which an ion exchange group can easily beintroduced would include such groups as oxy acids, salts, or esters orcarbon, nitrogen, silicon, phosphorous, sulfer chlorine, arsenic,selenium, or tellurium.

The NAFION material along with any filler materials used may be appliedby any method which will result in a thin uniform film as required aboveto form an adequate membrane over top of the deposited mating material.Among the methods thought to be suitable would be: deposition from aslurry, drawing the material onto the surface of the matting materialwith a vacuum, pouring the slurry over the matting material, brushing ona solution, or spraying in some fashion such as by a plasma spray.Vacuum forming from a slurry may be the most economical method since theequipment used for such a method would be the same as that used to applythe asbestos diaphragms.

A typical slurry for deposition upon a foraminous cathode with the matin place can be made by using NAFION particulate material with a 1208equivalent weight or 1073 equivalent weight mixed with a suitablesolvent such as 1,1,2-trichlorotrifluoroethane which is available fromE. I. duPont deNemours & Co. under the trademark FREON 113. FREON 113works well because it softens the NAFION particles thus making it easierto reduce the particle size by shearing to yield a very uniform NAFIONdispersion. It is also believed an aqueous slurry of NAFION with analkylarylpolyether alcohol available from Diamond Shamrock Corp. underthe trademark HYONIC PE260 used as a wetting agent would also performwell. The NAFION material is in the sulfonyl fluoride or thermoplasticform which unlike the sodium and acid forms is completely fusible into apolymeric film. A typical method for making such a suspension would beto utilize a stirrer fitted with a jacketed chamber and a refluxcondenser into which NAFION particulate material is added along withFREON 113 solvent. The system is heated with hot water to boil the FREON113 solvent and cold water is run through the reflux condenser tocondense the FREON 113 solvent. Refluxing of the NAFION and FREON 113mixture for approximately 10 minutes and then shearing for 30 minuteswhile continuing to reflux the FREON 113 solvent, produces a good NAFIONslurry from which to deposit a film onto the mating material surface.Various thermoplastic materials compatible with NAFION may be used asfillers in the slurry to reduce the cost while producing a good film.Examples of such fillers would be a fluorinated ethylene propylenecopolymer or a perfluoroalkoxy material.

The NAFION slurry may be applied to the support structure in variousways, the object being to produce a continuous uniform film afterfusing. Subsequent to application the FREON 113 is allowed to evaporateand then the particulate material is fused into a film. This isaccomplished by baking the entire foraminous electrode assembly in anoven generally at a temperature in the range of 240° to 300° C. Morethan one application and subsequent fusion of a thermoplastic materialslurry may be necessary in order to produce hole free continuous film.

Once a thin and uniform film is formed on the surface of the mattingmaterial which is substantially impermeable to hydraulic flow, the filmmay then be hydrolyzed into the infusible ion exchange sulfonic acidform. Hydrolyzing or saponifying of the NAFION is a fairly simpleprocedure for the conversion of a sulfonyl fluoride form to the freeacid form. This may be accomplished by soaking the film in a sodiumhydroxide solution, sodium hydroxide in dimethyl sulfoxide solution,potassium hydroxide solution, or potassium hydroxide in dimethylsulfoxide solution. Any of these treatments appear to work equally wellalthough different temperatures and times are required to accomplish thehydrolysis. Once this has been accomplished, the electrode is then readyfor use in a standard diaphragm electrolytic cell. The conditions of thecell should be altered to operate the cell as a membrane electrolyticcell.

Various means of improving these substrate materials have been sought,one of the most effective of which is the surface chemical treatment ofthe substrate itself. Generally these treatments consist of reacting thesulfonyl fluoride pendent groups with substances which will yield lesspolar bonding and thereby absorb fewer water molecules by hydrogenbonding. This has a tendency to narrow the pore openings through whichthe cations travel so that less water of hydration is transmitted withthe cations through the membrane. An example of this would be to reactthe ethylene diamine with the pendent groups in the sulfonyl fluorideform to tie two of the pendent groups together by two nitrogen atoms inthe ethylene diamine. Generally, in a film thickness of 7 mils, thesurface treatment will be done to a depth of approximately 2 mils on oneside of the film by controlling the time of reaction. This will resultin good electrical conductivity and cation transmission with lesshydroxide ion and associated water reverse migration.

The resultant membrane electrolytic cell will yield a high currentdensity, a lower sodium chloride concentration in the resultant sodiumhydroxide solution compared to a standard diaphragm cell, a higherresultant sodium hydroxide concentration, good utilization of existingcell space, longer lifetimes for the cell and a lower potential. Thus,those skilled in the art will recognize the advantages of the presentinvention to the chlorine and caustic industry.

In order that those skilled in the art may more readily understand thepresent invention and certain preferred aspects by which it may bepracticed, the following specific examples are afforded.

EXAMPLE 1

For testing in a laboratory cell, a membrane was formed over an asbestosmatting using a Buchner funnel as a framework structure over which toform the matting material. The matting material consisted of asbestosplus 5% fluorinated ethylene propylene copolymer as a binder in water toform a suspension. A vacuum was pulled on the Buchner funnel to draw thematting material onto the foraminous surface of the funnel until theporosity was reduced so as to capture particles in the size range of onemicron. A thermoplastic material slurry was made from a 1208 equivalentweight NAFION resin particle material mixed with FREON 113 solvent(1,1,2-trichlorotrifluoroethane). The mixture was stirred at 47.8° C. toobtain a 10 gram slurry. The slurry was then filtered through thematting material overtop of the Buchner funnel. Fusion of the NAFIONresin particles was accomplished by baking the matting material with theNAFION particles on the surface at a temperature of 300° C. for 60minutes. This resulted in a thin and uniform film which wassubstantially impermeable to hydraulic flow. Thereafter the NAFION washydrolyzed in a solution of dimethyl sulfoxide and water plus 10% sodiumhydroxide at a temperature of 70° C. for a time period of 70 minutes toconvert the sulfonyl fluroide form to the cation exchange sulfonic acidform.

This membrane over the matting material was then inserted into alaboratory test cell with a flow rate of 0.053 cc/min/sq.in. of cathodearea, a head of 14 inches of brine, a potential of 4.29 volts and acurrent of 6.28 amperes at 92° C. to achieve a current efficiency of 64%over a period of 37 days.

EXAMPLE 2

An asbestos matting was formed over a Buchner funnel as described inExample 1. A thermoplastic material slurry was made utilizing a 1073equivalent weight NAFION resin particle material in FREON 113 solvent(1,1,2-trichlorotrifluoroethane) and refluxed at 50° C. for 10 minutes.The resultant slurry was then poured over the asbestos mat. Afterevaporation of the FREON 113, the NAFION resin particles were fused at atemperature of 275° C. for 30 minutes into a thin and uniform film. Itwas then hydrolyzed in 35% sodium hydroxide for one week.

The resultant membrane over the matting material was then inserted intoa laboratory test cell under conditions according to Example 1 toachieve a maximum current efficiency of 50%.

EXAMPLE 3

An asbestos slurry was formed over a Buchner funnel according toExample 1. A slurry of thermoplastic material was made of 1073equivalent weight NAFIOn resin particle material in FREON 113 solventand refluxed according to Example 2. The slurry was then drawn onto thematting material with a 26 inch vacuum. The NAFION resin particles werefused at a temperature of 250° C. for 30 minutes, a second coat of theslurry was added by eye dropper to close holes in the film and asubsequent fusion was performed. The material was then hydrolyzed indimethyl sulfoxide and sodium hydroxide at a temperature for 80° for 70minutes.

The resultant membrane on the matting material was then inserted into alaboratory test cell under conditions according to Example 1 to achievea 53% current efficiency for a period of 78 days.

EXAMPLE 4

A matting material was applied over a Buchner funnel and dried at 150°C. for 45 minutes as described in Example 1. A thermoplastic materialslurry was made according to Example 1 and applied in a series of fourcoats the first two of which were brushed over the entire surface, thethird and fourth were partial coatings to cover up holes. The film wasfused at a temperature of 250° C. for thirty minutes and air tested forleaks after fusing each coat. The resultant film was then hydrolyzed ina solution of dimethyl sulfoxide and sodium hydroxide at a temperatureof 80° C. for a period of 70 minutes.

The resultant membrane was then placed into a laboratory cell fortesting under conditions according to Example 1 to achieve a currentefficiency of approximately 60% for a time period of 67 days.

Thus it should be apparent from the foregoing description of thepreferred embodiments that the method herein described accomplishes theobjects of the invention and solves the problems attendant to conversionof diaphragm electrolytic cells to membrane electrolytic cells withoutthe substantial capital costs associated with prior methods ofconversion.

What is claimed is:
 1. A membrane separator for a standard diaphragmelectrolytic cell comprising: a standard diaphragm cell foraminouselectrode; on the surface of said standard diaphragm cell foraminouselectrode, a layer of matting material of such thickness as tosubstantially reduce the porosity of the standard diaphragm foraminouselectrode; and on the surface of said matting material, a thin anduniform hydraulically impermeable cation exchange membrane consistingessentially of a film of copolymer having the repeating structure unitsof the formula: ##STR7## wherein R represents the group ##STR8## inwhich R¹ is fluorine or perfluoroalkyl of 1 to 10 carbon atoms; Y isfluorine or trifluoromethyl; m is 1, 2 or 3; n is O or 1; X is fluorine,chlorine, or trifluoromethyl; X¹ is X or CF₃ --CF₂ --_(a) O --; a is Oor integer from 1 to 5; and wherein R² is an ion exchange group selectedfrom the group of oxy acids, salts or esters of carbon, nitrogen,silicon, phosphorous, sulfur, chlorine, arsenic, selenium or tellurium.2. A membrane separator according to claim 1 wherein said membranematerial is surface treated to improve the selectivity of said membranematerial.